Johnston's Harbour
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Johnston's Harbour clear sky chart from 2005-11-06 to 2025-07-21.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	14% 12% 4 9% 5% 5% 4 3 5% 6% 32%	119916
	Night	15% 13% 5% 12% 6% 5% 5% 3 5% 5% 26%	119286
Transparency	Day	33% 40% 7% 12% 7% 1	123488
	Night	38% 39% 6% 10% 6% 1	123874
Seeing	Day	20% 9% 25% 28% 15% 3	16166
	Night	22% 11% 26% 26% 13% 2	25381

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 21% 10% 15% 13% 7% 3 4 3 3 8%	3183
	11th to 20th	Day	11% 21% 11% 27% 11% 5% 4 2 2 2 4	3589
	21st to end	Day	14% 18% 10% 19% 7% 6% 5% 4 4 4 9%	4396
February	1st to 10th	Day	13% 21% 6% 17% 9% 5% 5% 3 4 4 14%	1800
	11th to 20th	Day	11% 17% 6% 12% 8% 6% 6% 3 6% 5% 18%	1753
	21st to end	Day	12% 19% 7% 12% 6% 6% 5% 3 5% 5% 20%	1620
March	1st to 10th	Day	16% 11% 4 9% 5% 5% 5% 4 5% 6% 30%	4743
	11th to 20th	Day	11% 15% 6% 11% 6% 5% 5% 4 5% 6% 26%	4980
	21st to end	Day	18% 13% 6% 8% 5% 4 4 3 4 5% 29%	5465
April	1st to 10th	Day	17% 13% 4 7% 5% 4 4 4 5% 6% 31%	2326
	11th to 20th	Day	18% 10% 3 6% 4 4 3 3 4 6% 40%	2572
	21st to end	Day	18% 10% 4 6% 4 3 4 4 5% 7% 36%	2600
May	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 5% 7% 41%	5888
	11th to 20th	Day	11% 7% 3 7% 4 5% 4 3 5% 7% 43%	6105
	21st to end	Day	17% 8% 2 6% 3 4 4 3 5% 7% 41%	7009
June	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 7% 41%	3000
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 4 6% 9% 43%	3000
	21st to end	Day	13% 7% 3 6% 4 4 4 4 6% 8% 41%	3000
July	1st to 10th	Day	12% 7% 3 6% 5% 4 4 4 6% 8% 40%	4910
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 5% 8% 52%	4818
	21st to end	Day	9% 5% 1 4 3 3 4 4 6% 8% 53%	4561
August	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 7% 10% 44%	4104
	11th to 20th	Day	15% 6% 2 5% 3 4 4 4 6% 9% 43%	4044
	21st to end	Day	8% 6% 2 6% 4 5% 5% 4 6% 9% 46%	4532
September	1st to 10th	Day	12% 7% 2 7% 4 5% 5% 3 5% 7% 43%	2244
	11th to 20th	Day	12% 5% 2 6% 3 4 5% 4 6% 8% 46%	2090
	21st to end	Day	17% 8% 2 7% 4 5% 5% 3 5% 7% 37%	2090
October	1st to 10th	Day	16% 10% 4 7% 5% 5% 4 4 6% 6% 33%	1940
	11th to 20th	Day	24% 13% 4 9% 5% 5% 5% 3 4 6% 22%	1900
	21st to end	Day	25% 16% 5% 8% 5% 5% 4 2 5% 6% 19%	1981
November	1st to 10th	Day	18% 16% 5% 10% 5% 5% 6% 3 5% 6% 21%	1727
	11th to 20th	Day	18% 17% 5% 12% 7% 5% 4 4 5% 6% 17%	1600
	21st to end	Day	21% 22% 7% 12% 7% 5% 5% 3 4 4 10%	1600
December	1st to 10th	Day	15% 29% 9% 15% 6% 5% 4 3 4 3 7%	2600
	11th to 20th	Day	20% 24% 7% 18% 7% 4 3 3 3 3 8%	2670
	21st to end	Day	26% 24% 8% 17% 5% 6% 2 2 3 3 5%	3476
January	1st to 10th	Night	13% 18% 9% 20% 13% 8% 5% 2 3 2 7%	6780
	11th to 20th	Night	12% 19% 8% 22% 11% 7% 5% 3 3 3 7%	6316
	21st to end	Night	12% 17% 8% 19% 8% 7% 6% 4 4 4 12%	6560
February	1st to 10th	Night	11% 21% 6% 17% 8% 5% 4 4 5% 5% 15%	2600
	11th to 20th	Night	13% 17% 6% 13% 7% 5% 5% 3 5% 5% 20%	2533
	21st to end	Night	14% 15% 6% 12% 6% 5% 5% 4 5% 5% 24%	2005
March	1st to 10th	Night	14% 11% 4 10% 6% 5% 5% 3 5% 7% 31%	5097
	11th to 20th	Night	11% 12% 4 9% 5% 5% 5% 4 6% 7% 32%	4979
	21st to end	Night	19% 11% 5% 9% 5% 5% 4 3 4 4 31%	5467
April	1st to 10th	Night	20% 11% 3 8% 5% 3 4 3 5% 5% 33%	2091
	11th to 20th	Night	18% 8% 3 6% 4 3 5% 4 5% 7% 36%	1777
	21st to end	Night	19% 11% 3 8% 5% 4 4 4 5% 6% 33%	1800
May	1st to 10th	Night	14% 7% 3 8% 5% 5% 5% 4 5% 7% 35%	4053
	11th to 20th	Night	10% 8% 3 8% 4 5% 4 3 6% 8% 42%	3787
	21st to end	Night	18% 8% 3 6% 4 4 5% 3 4 6% 38%	3865
June	1st to 10th	Night	12% 9% 2 7% 6% 4 5% 4 5% 8% 40%	1400
	11th to 20th	Night	9% 6% 3 6% 5% 5% 5% 5% 6% 8% 43%	1400
	21st to end	Night	12% 7% 3 5% 5% 5% 5% 5% 5% 9% 40%	1400
July	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 5% 8% 47%	2280
	11th to 20th	Night	11% 5% 2 6% 4 4 3 3 6% 7% 50%	2369
	21st to end	Night	7% 6% 3 5% 4 4 4 3 6% 8% 51%	2996
August	1st to 10th	Night	9% 7% 3 6% 4 4 4 4 6% 7% 45%	2825
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 4 6% 8% 41%	2785
	21st to end	Night	9% 7% 2 6% 4 4 4 4 5% 7% 47%	3124
September	1st to 10th	Night	11% 5% 1 6% 4 4 5% 3 6% 7% 47%	1914
	11th to 20th	Night	11% 5% 2 5% 4 5% 4 3 6% 6% 48%	2090
	21st to end	Night	16% 9% 3 7% 4 3 4 4 5% 7% 37%	2090
October	1st to 10th	Night	17% 8% 4 9% 5% 5% 5% 3 4 6% 33%	2239
	11th to 20th	Night	22% 12% 3 10% 6% 5% 4 3 5% 6% 24%	2280
	21st to end	Night	23% 14% 5% 10% 6% 5% 5% 3 4 5% 20%	2612
November	1st to 10th	Night	20% 14% 4 10% 5% 5% 4 4 5% 5% 25%	2571
	11th to 20th	Night	16% 18% 6% 13% 6% 4 5% 3 4 6% 18%	2800
	21st to end	Night	22% 21% 5% 12% 8% 5% 4 3 4 4 12%	2800
December	1st to 10th	Night	16% 23% 8% 16% 8% 6% 5% 4 3 3 8%	4580
	11th to 20th	Night	15% 21% 9% 19% 8% 6% 5% 3 4 3 7%	5519
	21st to end	Night	22% 23% 7% 16% 7% 8% 3 2 3 3 6%	7502

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	62% 28% 3 5% 2 1	3184
	11th to 20th	Day	69% 26% 1 4 1	3620
	21st to end	Day	57% 30% 5% 5% 3	4396
February	1st to 10th	Day	29% 63% 3 5% 1	1800
	11th to 20th	Day	25% 61% 6% 4 3	1762
	21st to end	Day	25% 62% 4 6% 3	1620
March	1st to 10th	Day	42% 30% 10% 6% 11% 2	4746
	11th to 20th	Day	44% 29% 10% 13% 4	4980
	21st to end	Day	41% 31% 11% 13% 4 1	5465
April	1st to 10th	Day	23% 61% 4 8% 4	2326
	11th to 20th	Day	19% 62% 3 10% 6% 1	2572
	21st to end	Day	19% 62% 5% 8% 5% 1	2600
May	1st to 10th	Day	30% 29% 10% 14% 13% 3	5897
	11th to 20th	Day	30% 32% 9% 13% 15% 1	6109
	21st to end	Day	35% 29% 8% 12% 12% 3	7009
June	1st to 10th	Day	15% 63% 3 11% 7% 1	3000
	11th to 20th	Day	18% 62% 3 10% 6% 1	3000
	21st to end	Day	17% 61% 4 10% 7% 1	3000
July	1st to 10th	Day	32% 35% 10% 14% 8% 1	4836
	11th to 20th	Day	20% 35% 9% 27% 9%	4737
	21st to end	Day	19% 35% 7% 24% 14% 1	4417
August	1st to 10th	Day	23% 36% 9% 21% 11% 1	4091
	11th to 20th	Day	29% 37% 11% 17% 6%	4083
	21st to end	Day	23% 37% 9% 25% 6%	4531
September	1st to 10th	Day	16% 62% 6% 10% 5%	2244
	11th to 20th	Day	12% 61% 4 15% 7%	2090
	21st to end	Day	16% 61% 7% 11% 4	2090
October	1st to 10th	Day	24% 37% 6% 19% 13%	3226
	11th to 20th	Day	31% 37% 8% 19% 5%	3140
	21st to end	Day	38% 36% 9% 13% 4	3244
November	1st to 10th	Day	31% 57% 4 5% 2	1727
	11th to 20th	Day	29% 59% 5% 5% 1	1600
	21st to end	Day	33% 59% 4 3 1	1600
December	1st to 10th	Day	52% 36% 8% 3 1	2600
	11th to 20th	Day	58% 32% 6% 4 1	2670
	21st to end	Day	65% 28% 3 4 1	3476
January	1st to 10th	Night	63% 27% 3 5% 2	6780
	11th to 20th	Night	61% 27% 3 6% 3	6360
	21st to end	Night	55% 30% 5% 5% 4 1	6560
February	1st to 10th	Night	27% 63% 3 5% 2	2600
	11th to 20th	Night	23% 62% 3 9% 3	2535
	21st to end	Night	23% 62% 4 7% 4	2005
March	1st to 10th	Night	38% 30% 10% 10% 10% 2	5107
	11th to 20th	Night	36% 30% 13% 14% 5% 1	4980
	21st to end	Night	42% 33% 4 11% 9% 1	5467
April	1st to 10th	Night	25% 62% 2 7% 4	2091
	11th to 20th	Night	19% 62% 3 8% 7% 1	1777
	21st to end	Night	21% 63% 4 6% 6% 1	1800
May	1st to 10th	Night	35% 31% 5% 12% 12% 5%	4060
	11th to 20th	Night	30% 30% 8% 17% 14% 1	3788
	21st to end	Night	38% 30% 5% 16% 9% 2	3866
June	1st to 10th	Night	13% 62% 3 11% 10% 1	1400
	11th to 20th	Night	16% 61% 5% 10% 8% 1	1400
	21st to end	Night	15% 61% 5% 10% 7% 1	1400
July	1st to 10th	Night	28% 35% 9% 14% 9% 4	2245
	11th to 20th	Night	22% 34% 5% 28% 10%	2327
	21st to end	Night	22% 35% 6% 16% 20% 1	2900
August	1st to 10th	Night	25% 36% 8% 20% 11% 1	2814
	11th to 20th	Night	31% 38% 10% 13% 7%	2812
	21st to end	Night	23% 37% 7% 24% 9%	3124
September	1st to 10th	Night	14% 61% 7% 13% 6%	1914
	11th to 20th	Night	13% 61% 5% 13% 8%	2090
	21st to end	Night	17% 61% 6% 11% 5%	2090
October	1st to 10th	Night	22% 36% 4 23% 14%	3733
	11th to 20th	Night	29% 37% 10% 18% 5%	3779
	21st to end	Night	33% 37% 11% 15% 3	4299
November	1st to 10th	Night	30% 57% 4 7% 1	2571
	11th to 20th	Night	29% 59% 6% 4 2	2800
	21st to end	Night	34% 59% 3 2 1	2800
December	1st to 10th	Night	50% 36% 9% 3 1	4580
	11th to 20th	Night	57% 32% 5% 3 3	5518
	21st to end	Night	64% 27% 5% 3 1	7502

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	24% 16% 32% 20% 6% 1	207
	11th to 20th	Day	23% 14% 35% 20% 8%	300
	21st to end	Day	28% 11% 28% 25% 8%	330
February	1st to 10th	Day	25% 22% 30% 17% 6%	300
	11th to 20th	Day	22% 17% 32% 24% 4	300
	21st to end	Day	24% 22% 34% 18% 2	249
March	1st to 10th	Day	20% 11% 36% 25% 7%	410
	11th to 20th	Day	22% 12% 31% 27% 7% 1	500
	21st to end	Day	26% 9% 29% 28% 7% 1	550
April	1st to 10th	Day	22% 10% 30% 28% 8% 1	500
	11th to 20th	Day	27% 10% 26% 21% 12% 4	492
	21st to end	Day	24% 10% 26% 27% 10% 1	500
May	1st to 10th	Day	21% 7% 27% 31% 12% 2	503
	11th to 20th	Day	18% 5% 21% 31% 20% 5%	527
	21st to end	Day	14% 4 21% 35% 20% 6%	594
June	1st to 10th	Day	13% 4 25% 37% 18% 3	730
	11th to 20th	Day	13% 5% 25% 35% 18% 3	730
	21st to end	Day	18% 6% 22% 34% 18% 3	730
July	1st to 10th	Day	10% 4 19% 37% 27% 3	721
	11th to 20th	Day	12% 4 22% 34% 22% 6%	718
	21st to end	Day	12% 2 21% 34% 26% 4	588
August	1st to 10th	Day	13% 6% 17% 31% 27% 6%	486
	11th to 20th	Day	14% 3 13% 33% 29% 9%	479
	21st to end	Day	17% 6% 20% 28% 23% 6%	539
September	1st to 10th	Day	18% 5% 21% 27% 23% 7%	490
	11th to 20th	Day	11% 6% 18% 28% 29% 8%	490
	21st to end	Day	19% 8% 22% 25% 22% 5%	490
October	1st to 10th	Day	24% 12% 22% 21% 16% 4	490
	11th to 20th	Day	27% 15% 29% 22% 6% 1	490
	21st to end	Day	33% 15% 26% 20% 4 1	430
November	1st to 10th	Day	27% 14% 34% 17% 7%	283
	11th to 20th	Day	27% 18% 34% 18% 3 1	200
	21st to end	Day	28% 21% 35% 15% 2	200
December	1st to 10th	Day	29% 21% 29% 15% 7%	200
	11th to 20th	Day	29% 25% 26% 18% 3	200
	21st to end	Day	32% 20% 35% 10% 2 1	220
January	1st to 10th	Night	24% 19% 30% 20% 6% 1	1000
	11th to 20th	Night	27% 16% 29% 22% 6% 1	1000
	21st to end	Night	22% 12% 32% 25% 9% 1	911
February	1st to 10th	Night	19% 18% 36% 22% 5% 1	800
	11th to 20th	Night	22% 11% 34% 25% 7% 1	783
	21st to end	Night	21% 12% 37% 24% 5%	660
March	1st to 10th	Night	18% 11% 34% 29% 8% 1	800
	11th to 20th	Night	18% 13% 33% 26% 9% 1	800
	21st to end	Night	21% 6% 29% 31% 12% 1	880
April	1st to 10th	Night	25% 10% 28% 29% 7% 1	775
	11th to 20th	Night	22% 10% 25% 28% 12% 3	593
	21st to end	Night	24% 9% 24% 28% 14% 2	600
May	1st to 10th	Night	17% 3 24% 36% 18% 2	600
	11th to 20th	Night	17% 3 16% 38% 23% 2	489
	21st to end	Night	16% 3 18% 38% 21% 4	433
June	1st to 10th	Night	16% 7% 20% 36% 18% 3	400
	11th to 20th	Night	14% 5% 19% 40% 20% 3	400
	21st to end	Night	13% 4 21% 33% 27% 2	400
July	1st to 10th	Night	8% 3 14% 47% 24% 4	390
	11th to 20th	Night	11% 2 19% 35% 30% 4	438
	21st to end	Night	9% 3 14% 41% 30% 3	624
August	1st to 10th	Night	11% 2 16% 36% 28% 6%	567
	11th to 20th	Night	12% 3 16% 29% 31% 9%	564
	21st to end	Night	14% 4 19% 31% 26% 5%	627
September	1st to 10th	Night	14% 5% 18% 32% 23% 9%	732
	11th to 20th	Night	13% 5% 20% 29% 26% 7%	760
	21st to end	Night	21% 8% 19% 22% 23% 7%	760
October	1st to 10th	Night	22% 9% 24% 24% 18% 3	760
	11th to 20th	Night	28% 13% 23% 25% 8% 2	760
	21st to end	Night	33% 13% 28% 21% 5% 1	836
November	1st to 10th	Night	28% 15% 25% 22% 8% 1	780
	11th to 20th	Night	26% 21% 26% 19% 7%	800
	21st to end	Night	34% 17% 28% 17% 4	800
December	1st to 10th	Night	33% 19% 29% 12% 7%	800
	11th to 20th	Night	28% 22% 32% 13% 4	959
	21st to end	Night	34% 15% 32% 15% 4	1100

Page generated at 2025-07-18 01:10:03. Version ac07

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